[Mesa-dev] [PATCH v3] i915: fixing driver crashes if too few vertices are submitted

[Ilia Mirkin]

On Fri, Sep 11, 2015 at 12:36 PM, Marius Predut <marius.predut at intel.com> wrote:
> Comparison with a signed expression and unsigned value
> is converted to unsigned value, reason for minus value is interpreted
> as a big unsigned value. For this case the "for" loop
> is going into unexpected behavior.
>
> v1: Brian Paul: code style fix.
> v2: Ian Romanick: glDrawArrays(GL_QUADS, 0, (n * 4) + k) fail , k < 4.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
> Signed-off-by: Marius Predut <marius.predut at intel.com>
> ---
>  src/mesa/tnl_dd/t_dd_dmatmp.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/src/mesa/tnl_dd/t_dd_dmatmp.h b/src/mesa/tnl_dd/t_dd_dmatmp.h
> index 7be3954..f99d977 100644
> --- a/src/mesa/tnl_dd/t_dd_dmatmp.h
> +++ b/src/mesa/tnl_dd/t_dd_dmatmp.h
> @@ -627,6 +627,13 @@ static void TAG(render_quads_verts)( struct gl_context *ctx,
>        LOCAL_VARS;
>        GLuint j;
>
> +      /* Page 18 (page 32 of the PDF) of the OpenGL 2.1 spec says:
> +       * The total number of vertices between Begin and End is 4n + k,
> +       * where 0 ≤ k ≤ 3; if k is not zero, the final k vertices are  ignored.
> +       */
> +      count = (count / 4) * 4;

Might be just me, but I'd find

count &= ~0x3

to be a lot clearer. Don't know if the compiler can make such an
optimization. However this seems wrong... you're supposed to draw
start..count, so that's the value that has to be div-by-4. Further up,
when there's native quad support, the logic does:

      /* Emit whole number of quads in total.  dmasz is already a multiple
       * of 4.
       */
      count -= (count-start)%4;

Which seems more accurate.

> +      if(count == 0) return;

if (count == 0)

note the space. That's the style used in all of mesa.

$ git grep '\sif(' | wc -l
1076
$ git grep '\sif (' | wc -l
58071

I guess a few 'if(' instances snuck through, mostly in src/gallium.
But the overwhelming majority are of the 'if (' style.

> +
>        INIT(GL_TRIANGLES);
>
>        for (j = start; j < count-3; j += 4) {
> --
> 1.9.1
>
>
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